1. Field of the Invention
The present invention relates to an optical disk apparatus, and more particularly, to an improved optical disk apparatus for optically recording an information signal in an optical disk and/or optically reproducing the information which is stored on the optical disk.
2. Description of Related Art
As computers become advanced, the quantity of programs and data to be used has increased, which has made it necessary to provide for a file apparatus of a large capacity to store programs and data. While development of a large capacity file apparatus of a small size progresses, due to compatibility of a medium, further development of an optical disk apparatus, which has a large capacity, is expected in the future.
There are various types of optical disks. Of the variety of optical disks, a CD-ROM is an optical disk exclusively for reading and is becoming popular as means for distributing software. Because of this, a recent tendency is that a CD-ROM driving apparatus is incorporated in a personal computer as standard equipment.
Meanwhile, a magneto optical disk (MO) and a phase-variable optical disk, which are erasable/writable optical disks, are rapidly becoming popular as a large capacity file for storing data created by a user and for the purpose of back-up.
A CD-ROM, a magneto optical disk and a phase-variable optical disk cannot be driven unless each driven by a special driving apparatus therefor at present. However, if it is possible to drive both mediums with only one driving apparatus, that will be a great improvement in terms of costs and ease of use.
Japanese Patent Application Laid-Open No. 4-85760 (1992) and Japanese Patent Application Laid-Open No. 5-159409 (1993) propose disk apparatuses which can drive a CD-ROM, a magneto optical disk and a phase-variable optical disk with only one driving apparatus. Japanese Patent Application Laid-Open No. 4-85760 (1992) mainly uses an operation processing circuit to realize this, while Japanese Patent Application Laid-Open No. 5-159409 (1993) uses a three beam method and a push-pull method for detection of a tracking error signal and a Foucault method for detection of a focus error signal.
Further, with respect to optical disk apparatuses in general, while it is necessary to reduce the size of an optical system, namely, the most important part of the apparatus, to reduce costs and the size, use of a hologram optical element is becoming popular for the purpose of deflection and separation of a light beam. Use of a hologram optical element requires a caution since when a light beam is deflected in a predetermined direction, another light beam is always created in an unnecessary direction. If a light beams is created in an unnecessary direction, the efficiency of utilizing light is deteriorated to reduce an output current from a photodetector. As a result, the reliability of the apparatus is deteriorated.